Place the sliced onions and bitter gourds with 1/4 cup of water, a pinch of salt and turmeric powder in a microwave safe bowl with a lid (with small vent for steam). Nuke for 5-6 minutes at HIGH. Check if it is tender at the end of 6 minutes. If not, microwave for some more time until bitter gourd slices are soft.
Heat the oil in a pan. Once the oil is hot enough, pop the cumin seeds, mustard seeds and curry leaves. Add the ginger and garlic, saute for 30 seconds.
Add the microwave cooked bitter gourds-onions to the pan. Put in the chopped tomatoes, chopped ripe mango, curry powder, salt, red chilli powder. Saute them together well and cover the pan. Let this simmer for 5-7 minutes for all the flavours to come together.
Check for salt and take the curry off the flame. Squeeze some fresh lemon juice on the top and serve hot with Phulkas or a simple khichdi.